Method of manufacture of chalcogenide-based photovoltaic cells

ABSTRACT

The invention is a method of forming a cadmium sulfide based buffer on a copper chalcogenide based absorber in making a photovoltaic cell. The buffer is sputtered in two steps the first being at low rates or relatively high pressures and the second at high rates or relatively low pressures. The resulting cell has good efficiency and according to one embodiment is characterized by a narrow interface between the absorber and buffer layers. The buffer is further characterized according to a second embodiment by a relatively high oxygen content.

FIELD OF THE INVENTION

This invention relates to a method of manufacture of chalcogenide-basedphotovoltaic cells and particularly to a method of forming a bufferlayer in such cells and the cells made by this method.

BACKGROUND OF THE INVENTION

Photovoltaic cells can be made using p-type chalcogenide based materialsas absorber layers which convert incident light or radiation toelectrical energy. These p-type chalcogenides are typically selenides,sulfides or sulfide selenides of at least one, and more typically atleast two or three of the following metals: Cu, In, Ga, Al (referred toherein as CIS, CISS, CIAS, CIASS, CIGS, CIGSS, or CIAGSS depending uponthe combination of elements used). Using a CdS based buffer layer nearor adjacent to the p-type chalcogenide is also known.

It is known that CdS layers can be formed on various substrates bychemical bath deposition, physical vapor deposition or sputtering. Seee.g. Abou-Ras et al (Thin Solid Films 480-481 (2005) 118-123) and U.S.Pat. No. 5,500,055. Abou-Ras specifically looked at the effect ofdeposition method comparing CBD deposition to PVD deposition andobserved that CBD deposition of the CdS created more efficient cellscompared with cells made with PVD. Abou-Ras proposes that the lack ordecrease of interdiffusion at the CIGS-CdS interface in the case of PVDdeposited cells is a reason for their decreased efficiencies.

SUMMARY OF THE INVENTION

Applicants have surprisingly found that sputtering buffer material suchas CdS onto the p-type chalcogenide at relatively high pressures and/orrelatively low rates in a substantially inert atmosphere leads to lessinterdiffusion with the underlying chalcogenide absorber than doessputtering at the traditional low pressure sputtering conditions butalso leads to higher cell efficiencies than does the traditional lowpressure sputtering. This is particularly unexpected in view ofAbou-Ras's teaching about the benefit of interdiffusion. Applicants havefurther discovered that surprisingly the benefits of sputtering at thelow rates and/or high pressure are obtained even when only a thininterfacial layer of the buffer is formed at the high pressure and theremainder of the buffer material is formed at higher rates, lowerpressure or higher power conditions.

Thus, according to one embodiment, the invention is a method comprisingforming a chalcogenide-based absorber layer on a substrate, forming afirst portion of a buffer layer comprising cadmium and sulfur on theabsorber by sputtering in an inert atmosphere at a low pressure and arate of formation of less than 10 nm/min to form a layer having athickness less than 20 nm, preferably less than 15 nm and at least 1 nm.Preferably this is accomplished using a working pressure of from 0.08 to0.12 mbar (0.06 to 0.09 torr or 8-12 Pa) and/or at low sputtering power.After forming the first portion, the method further comprises forming asecond portion of the buffer layer by sputtering at a rate of at least20 nm/min to form a second portion having a thickness of at least 10 nm.Preferably, the second portion is formed at a working pressure of lessthan 0.05 mbar (0.04 torr or 5 Pa), preferably less than 0.01 (0.008torr or 1 Pa), more preferably less than 0.005 mbar (0.004 torr or 0.5Pa). However, the rate may also be increased by increasing thesputtering power. Exact power settings required for each step will varydepending upon the sputtering tool being used.

This invention is also a photovoltaic cell made by the preceding method.The cells made by this method are characterized by interdiffusionbetween the CdS and absorber layer. The interdiffusion interface regionis defined at the absorber region by the point at which the atomicfraction of cadmium exceeds 0.05 in energy dispersive x-ray spectroscopy(EDS) scans of cross section of the cell and defined at the bufferregion by the point at which the atomic fraction of indium and seleniumis less than 0.05 and preferably the atomic fraction of copper is lessthan 0.10. The atomic fraction is based on total atomic amounts ofcopper, indium, gallium, selenium, cadmium, sulfur, and oxygen. Thegrain size of the cadmium sulfide grains preferably is less than 50 nm,more preferably less than 30 nm, and most preferably less than 20 nm.Thus, according to one embodiment the invention is a photovoltaic cellcomprising a backside electrode (also referred to herein as a backsideelectrical contact or backside electrical collector), achalcogenide-based absorber in contact with the backside electrode, acadmium sulfide based buffer layer on the absorber, a transparentconductive layer located at the opposite side of the buffer layer fromthe absorber layer, an electrical collector on the transparentconductive layer, wherein the cell has an interface between the absorberand the buffer of less than 10 nm thickness and the buffer preferablyhas an average grain size of less than 50 nm.

According to another embodiment this invention is a photovoltaic cellcomprising a backside electrode, a chalcogenide based absorber incontact with the backside electrode, a cadmium sulfide based bufferlayer on the absorber, a transparent conductive layer located at theopposite side of the buffer layer from the absorber layer, an electricalcollector on the transparent conductive layer, wherein the atomicfraction of oxygen in the first portion of the cadmium sulfide basedbuffer layer is at least 0.20 and in the second portion is less than0.20, preferably less than 0.15, more preferably less than 0.10.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a cross section of a representative cellaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of this invention includes forming a chalcogenide basedabsorber layer (absorbs electromagnetic radiation at relevantwavelengths and converts to electrical energy) on a substrate and thenforming a cadmium sulfide buffer layer on that absorber. When the methodis used to form a photovoltaic device, additional layers as are known inthe photovoltaic arts also will typically be added. For example, thesubstrate will typically include or bear backside electrical contacts. Atransparent conductive layer will be found above the buffer and anelectrical collector system (e.g. a grid) will typically be locatedabove the transparent conductive layer. An optional window layer may beused and protective layers may be applied over the transparentconductive layer and/or the electrical collector. With the exception ofthe cadmium sulfide based buffer layer, these additional layers may beformed by any method known in the art.

FIG. 1 shows one embodiment of a photovoltaic article 10 that may bemade by processes of the invention. This article 10 comprises asubstrate incorporating a support 22, a backside electrical contact 24,and a chalcogenide absorber 20. The article 10 further includes a bufferregion 28 incorporating an n-type chalcogenide composition of thepresent invention, an optional front side electrical contact windowregion 26, a transparent conductive region 30, a collection grid 40, andan optional barrier region 34 to help protect and isolate the article 10from the ambient. The buffer region has an interfacial area 28 asputtered at relatively high pressure and an additional region 28 bformed at lower pressure. Each of these components is shown in FIG. 1 asincluding a single layer, but any of these independently can be formedfrom multiple sublayers as desired. Additional layers (not shown)conventionally used in photovoltaic cells as presently known orhereafter developed may also be provided. As used occasionally herein,the top 12 of the cell is deemed to be that side which receives theincident light 16. The method of forming the cadmium sulfide based layeron the absorber can also be used in tandem cell structures where twocells are built on top of each other, each with an absorber that absorbsradiation at different wavelengths.

The support 22 may be a rigid or flexible substrate. Support 22 may beformed from a wide range of materials. These include glass, quartz,other ceramic materials, polymers, metals, metal alloys, intermetalliccompositions, paper, woven or non-woven fabrics, combinations of these,and the like. Stainless steel is preferred. Flexible substrates arepreferred to enable maximum utilization of the flexibility of the thinfilm absorber and other layers.

The backside electrical contact 24 provides a convenient way toelectrically couple article 10 to external circuitry in cases wheresupport 22 is non-conductive. Contact 24 may be formed from a wide rangeof electrically conductive materials, including one or more of Cu, Mo,Ag, Al, Cr, Ni, Ti, Ta, Nb, W, combinations of these, and the like.Conductive compositions incorporating Mo are preferred. The backsideelectrical contact 24 may also help to isolate the absorber 20 from thesupport 22 to minimize migration of support constituents into theabsorber 20. For instance, backside electrical contact 24 can help toblock the migration of Fe and Ni constituents of a stainless steelsupport 22 into the absorber 20. The backside electrical contact 24 alsocan protect the support 22 such as by protecting against Se if Se isused in the formation of absorber 20.

The chalcogenide absorber 20 preferably incorporates at least one p-typeGroup 16 chalcogenide, such as Group 16 selenides, sulfides, andselenides-sulfides that include at least one of copper, indium, and/orgallium. In many embodiments, these materials are present inpolycrystalline form. Advantageously, these materials exhibit excellentcross-sections for light absorption that allow absorber 20 to be verythin and flexible. In illustrative embodiments, a typical absorberregion 20 may have a thickness in the range from about 300 nm to about3000 nm, preferably about 1000 nm to about 2000 nm.

Representative examples of such p-type chalcogenides absorbers areselenides, sulfides, tellurides, and/or combinations of these thatinclude at least one of copper, indium, aluminum, and/or gallium. Moretypically at least two or even at least three of Cu, In, Ga, and Al arepresent. Sulfides and/or selenides are preferred. Some embodimentsinclude sulfides or selenides of copper and indium. Additionalembodiments include selenides or sulfides of copper, indium, andgallium. Aluminum may be used as an additional or alternative metal,typically replacing some or all of the gallium. Specific examplesinclude but are not limited to copper indium selenides, copper indiumgallium selenides, copper gallium selenides, copper indium sulfides,copper indium gallium sulfides, copper gallium sulfides, copper indiumsulfide selenides, copper gallium sulfide selenides, copper indiumaluminum sulfides, copper indium aluminum selenides, copper indiumaluminum sulfide selenide, copper indium aluminum gallium sulfides,copper indium aluminum gallium selenides, copper indium aluminum galliumsulfide selenide, and copper indium gallium sulfide selenides. Theabsorber materials also may be doped with other materials, such as Na,Li, or the like, to enhance performance. In addition, many chalcogenidematerials could incorporate at least some oxygen as an impurity in smallamounts without significant deleterious effects upon electronicproperties.

One preferred class of CIGS materials may be represented by the formula

Cu_(a)In_(b)Ga_(c)Al_(d)Se_(w)S_(x)Te_(y)Na_(z)  (A)

Wherein, if “a” is defined as 1, then:

-   “(b+c+d)/a”=1.0 to 2.5, preferably 1.0 to 1.65-   “b” is 0 to 2, preferably 0.8 to 1.3-   “c” is 0 to 0.5, preferably 0.05 to 0.35-   “d” is 0 to 0.5, preferably 0.05 to 0.35, preferably d=0-   “(w+x+y)” is 2 to 3, preferably 2 to 2.8-   “w” is 0 or more, preferably at least 1 and more preferably at least    2 to 3-   “x” is 0 to 3, preferably 0 to 0.5-   “y” is 0 to 3, preferably 0 to 0.5-   “z” is 0 to 0.5, preferably 0.005 to 0.02

The absorber 20 may be formed by any suitable method using a variety ofone or more techniques such as evaporation, sputtering,electrodeposition, spraying, and sintering. One preferred method isco-evaporation of the constituent elements from one or more suitabletargets, where the individual constituent elements are thermallyevaporated onto a hot surface coincidentally at the same time,sequentially, or a combination of these to form absorber 20. Afterdeposition, the deposited materials may be subjected to one or morefurther treatments to finalize the absorber properties.

Optional layers (not shown) may be used on the substrate in accordancewith conventional practices now known or hereafter developed to helpenhance adhesion between backside electrical contact 24 and the support22 and/or between backside electrical contact 24 and the absorber region20. Additionally, one or more barrier layers (not shown) also may beprovided over the backside of support 22 to help isolate device 10 fromthe ambient and/or to electrically isolate device 10.

The buffer region 28 is cadmium sulfide based material deposited in twosteps on the absorber 20. The first portion 28 a is formed by sputteringat a rate of less than 10 nm/min, preferably less than 8 nm/min and mostpreferably less than 5 nm/min to form a layer having a thickness lessthan 20 nm, preferably less than 15 nm and preferably at least 1 nm,more preferably at least 5 nm or by sputtering using a working pressureof at least 0.08 millibar (0.06 torr, 8 Pa), more preferably at least0.09 millibar (0.067 torr, 9 Pa), and most preferably about 0.1 millibar(0.075 torr, 10 Pa) and no greater than 0.12 millibar (0.09 torr, 12Pa), more preferably no greater than 0.11 millibar (0.083 torr, 11 Pa).Preferably, the atmosphere is inert or a sulfur containing gas, but ismost preferably inert. Power settings may be adjusted to achieve thedesired rate. Exact power settings required will vary depending upon thesputtering tool being used.

After forming the first portion, the method further comprises forming asecond portion the buffer layer by sputtering at a rate of at least 20nm/min to form a second portion having a thickness of at least 10 nm,preferably at least 15 nm, more preferably at least 20 nm, morepreferably still at least 30 nm, but preferably no more than 200 nm,more preferably no more than 100 nm, more preferably still no more than50 nm and most preferably no more than 30 nm. Preferably, the secondportion is formed at a working pressure of less than 0.05 mbar (0.04torr or 5 Pa), preferably less than 0.01 (0.008 torr or 1 Pa), morepreferably less than 0.005 mbar (0.004 torr or 0.5 Pa). However, therate may also be increased by increasing the power to the sputter tool.Exact power settings required for each step will vary depending upon thesputtering tool being used.

During such a deposition approach, the substrate is typically fixed toor otherwise supported upon a holder within the chamber such as bygripping components, or the like. However, the substrate may be orientedand affixed by a wide variety of means as desired. The substrate may beprovided in the chamber in a manner such that the substrate isstationary and/or non-stationary during the treatment. In someembodiments, for instance, the substrate can be supported on a rotatablechuck so that the substrate rotates during the deposition. In otherembodiments, the substrate may be a moving web.

One or more targets are operable provided in the deposition system. Thetargets are compositionally suitable to form the desired cadmium sulfidecomposition. For instance, to form n-type cadmium sulfide, a suitabletarget has a composition that includes cadmium- and sulfur-containingcompounds, and is preferably 99% pure cadmium and sulfur. Alternatively,a cadmium target can be used in the presence of a sulfur-containing gas.The resulting film comprising the first and second portions ispreferably at least 15 nanometers (nm), more preferably at least 20 andmore preferably still at least 30 nm but preferably no more than 200 nm,still more preferably up to 100 nm, yet more preferably up to 50 nm.

While a small amount of interdiffusion in cells made by the method ofthis invention may occur it is significantly less than is found when theatmosphere for sputtering is lower pressure. The interdiffusioninterface region is defined at the absorber region by the point at whichthe atomic fraction of cadmium exceeds 0.05 in energy dispersive x-rayspectroscopy scans of cross section of the cell and defined at thebuffer region by the point at which the atomic fraction of indium andselenium is less than 0.05 and preferably the atomic fraction of copperis less than 0.10. The atomic fraction is based on total atomic amountsof copper, indium, gallium, selenium, cadmium, sulfur, and oxygen. Thegrain size of the cadmium sulfide grains preferably is less than 50 nm,more preferably less than 30 nm, and most preferably less than 20 nm.According to this embodiment, the interface region is less than 10 nm,preferably less than 8 nm in thickness.

Atomic fraction can be determined from transmission electron microscope(TEM) line scans using energy dispersive x-ray spectroscopy (EDS).Samples for TEM analysis can be prepared by focused ion beam (FIB)milling using a FEI Strata Dual Beam FIB mill equipped with a Omniprobelift-out tool. TEM analysis can be performed for example on a FEI TecnaiTF-20XT FEGTEM equipped with a Fischione high angle annular dark field(HAADF) scanning TEM (STEM) detector and EDAX EDS detector. Theoperating voltage of the TEM can be level suitable for the equipment forexample voltages of about 200 keV are useful with the equipmentmentioned above.

Spatially-resolved EDS line scans can be acquired in STEM mode fromHAADF images. From the 100 nm long line scan, 50 spectra can be acquiredusing 2 nm spot-to-spot resolution and a STEM probe size of ˜1 nm. Thefull scale spectra (0-20 keV) can be converted into elementaldistribution profiles because peak intensity (number of integrated peakcounts after background removal) is directly proportional to theconcentration (in weight percent) of a certain element. Weight percentis then converted to atomic percentage based on mole weight of theelement. Note that as a skilled worker understands peak intensity iscorrected for the detector response and the other sample-dependentfactors. These adjustments are typically made based on manufacturer'sparameters for their EDS equipment or other suitable referencestandards. Grain size can be determined by standard analysis of TEMbright and dark field images.

The cadmium sulfide layer may contain a small amount of impurities. Thecadmium sulfide layer preferably consists essentially of cadmium,sulfur, oxygen and copper. Preferably the atomic fractions of cadmiumand sulfur are at least 0.3, the atomic fraction of oxygen is at least0.2 and the atomic fraction of copper is less than 0.15, more preferablyless than 0.10.

The atmosphere for sputtering is preferably an inert gas such as argon,helium or neon. The substrate may be placed at a predetermined distancefrom and orientation relative to the target(s). In some modes ofpractice, this distance can be varied during the course of thedeposition, if desired. Typically, the distance is in the range fromabout 50 millimeters (mm) to about 100 mm. Preferably, the distance isfrom about 60 mm to about 80 mm. Prior to starting deposition, thechamber typically is evacuated to a suitable base pressure. In manyembodiments, the base pressure is in the range from about 1×10⁻⁸ Torr toabout 1×10⁻⁶ Torr.

Conveniently, many modes of practice may be carried out at a temperaturein the range of from about 20° C. to about 30° C. Of course, cooler orwarmer temperatures may be used to help control deposition rate,deposition quality, or the like. The deposition may be carried out longenough to provide a layer of n-type material having a desired thickness,uniformity, and/or the like.

Optional window region 26, which may be a single layer or formed frommultiple sublayers, can help to protect against shunts. Window region 26also may protect buffer region 28 during subsequent deposition of theTCO region 30. The window region 26 may be formed from a wide range ofmaterials and often is formed from a resistive, transparent oxide suchas an oxide of Zn, In, Cd, Sn, combinations of these and the like. Anexemplary window material is intrinsic ZnO. A typical window region 26may have a thickness in the range from about 1 nm to about 200 nm,preferably about 10 nm to about 150 nm, more preferably about 80 toabout 120 nm.

The TCO region 30, which may be a single layer or formed from multiplesublayers, is electrically coupled to the buffer region 28 to provide atop conductive electrode for article 10. In many suitable embodiments,the TCO region 30 has a thickness in the range from about 10 nm to about1500 nm, preferably about 150 nm to about 200 nm. As shown, the TCOregion 30 is in direct contact with the window region 26, but one ormore intervening layers optionally may be interposed for a variety ofreasons such as to promote adhesion, enhance electrical performance, orthe like.

A wide variety of transparent conducting oxides; very thin conductive,transparent metal films; or combinations of these may be used in formingthe transparent conductive region 30. Transparent conductive oxides arepreferred. Examples of such TCOs include fluorine-doped tin oxide, tinoxide, indium oxide, indium tin oxide (ITO), aluminum doped zinc oxide(AZO), zinc oxide, combinations of these, and the like. In oneillustrative embodiment, TCO region 30 has a dual layer construction inwhich a first sublayer proximal to the buffer incorporates zinc oxideand a second sublayer incorporates ITO and/or AZO. TCO layers areconveniently formed via sputtering or other suitable depositiontechnique.

The optional electrical grid collection structure 40 may be depositedover the TCO region 30 to reduce the sheet resistance of this layer. Thegrid structure 40 preferably incorporates one or more of Ag, Al, Cu, Cr,Ni, Ti, Ta, TiN, TaN, and combinations thereof. Preferably the grid ismade of Ag. An optional film of Ni (not shown) may be used to enhanceadhesion of the grid structure to the TCO region 30. This structure canbe formed in a wide variety of ways, including being made of a wire meshor similar wire structure, being formed by screen-printing, ink-jetprinting, electroplating, photolithography, and metallizing thru asuitable mask using any suitable deposition technique.

A chalcogenide based photovoltaic cell may be rendered less susceptibleto moisture related degradation via direct, low temperature applicationof suitable barrier protection 34 to the top 12 of the photovoltaicarticle 10. The barrier protection may be a single layer or multiplesublayers. As shown the barrier does not cover the electrical gridstructure but a barrier that covered such grid may be used instead of orin addition to the barrier shown.

EXAMPLE

A photovoltaic cell is prepared as follows. A stainless steel substrateis provided. A niobium and molybdenum backside electrical contact isformed on the substrate by sputtering. A copper indium gallium selenideabsorber is formed by a 1-stage evaporation process where copper,indium, gallium and selenium are evaporated simultaneously from effusionsources onto the stainless steel substrate, which is held at ˜550° C.for ˜80 minutes. This process results in an absorber stoichiometry ofabout Cu(In_(0.8)Ga_(0.2))Se₂

A cadmium sulfide layer is radio frequency (rf) sputtered from a CdStarget (99.9+% purity) at 160 watts in the presence of argon. Thetemperature of the substrate is maintained at ≦35° C., and thetarget-to-substrate distance is ˜90 mm. This layer is deposited in twosteps, with the targeted thickness as set forth in Table 1. The firstdeposition on the absorber for all samples in FIG. 1 was deposited atabout 0.1 mbar (10 Pa), whereas the second deposition was done atpressures varying from 0.01 down to 0.002 mbar (1 to 0.2 Pa).

On the cadmium sulfide layer, i-ZnO and Al-doped ZnO layers weredeposited via rf-sputtering. A front side electrical collection grid(Ni:Al) was deposited on Al-doped ZnO via e-beam evaporation.

Cells made substantially according to the above procedure are tested forefficiency by measuring current-voltage characteristics underillumination. As shown in Table 1 cells made using a first portionformed at high pressure and a second portion at higher rate/lowerpressure has nearly equivalent performance as cells having an entirebuffer layer formed at high pressure but has the benefit of greatlyreducing the deposition time.

TABLE 1 Overall Deposition Deposition Portion 1 Portion 2 Average Maxrate, both 1 pressure 2 pressure target target Efficiency Efficiencyportions Sample (mbar) (mbar) thickness thickness (%) (%) (nm/min)Comaparative1 NA 0.002 NA 15 6.96 7.18 28 Comparative 2 NA 0.002 NA 1206.83 8.02 28 Comparative 3 ~0.1 NA 30 NA 9-10 10.61 5.6 (severalsamples) 4 ~0.1 0.002 15 15 8.94 9.57 9.5 5 ~0.1 0.01 15 15 7.37 9.489.3

What is claimed is:
 1. A method comprising forming a chalcogenide basedabsorber layer on a substrate, forming a first portion of a buffer layercomprising cadmium and sulfur on the absorber by sputtering at a rate ofless than 10 nm/min or by sputtering at a working pressure of from 8 to12 Pa to create a thickness of the first portion of 1 to 20 nm, andforming a second portion of the buffer layer comprising cadmium andsulfur by sputtering at a rate of at least 20 nm/min or a workingpressure of no more than 1 Pa wherein the thickness of the secondportion is at least 15 nm.
 2. The method of claim 1 wherein theatmosphere is inert.
 3. The method of claim 1 wherein sputtering is froma target of cadmium and sulfur or compounds thereof.
 4. The method ofclaim 1 wherein the second portion is sputtered at a rate of at least 30nm/min.
 5. The method of claim 1 wherein the second portion is sputteredat a pressure of no more than 1 Pa.
 6. The method of claim 1 wherein thefirst portion is sputtered at a rate of less than 8 nm/min.
 7. Themethod of claim 1 wherein the first portion is sputtered at a pressureof 8-12 Pa and the thickness of the first layer is 5 to 15 nm.
 8. Themethod of claim 1 wherein the substrate is flexible and bears a backsideelectrical contact.
 9. The method of claim 1 further comprising forminga transparent conductive layer over the buffer layer.
 10. The method ofclaim 9 further comprising forming a window layer between thetransparent conductive layer and the buffer layer.
 11. The method ofclaim 9 comprising forming an electrical connection grid on thetransparent conductive layer.
 12. The method of claim 9 comprisingproviding a barrier layer over the transparent conductive layer.
 13. Themethod of claim 11 comprising providing a barrier layer over thetransparent conductive layer and the grid.
 14. A photovoltaic cell madeby the method of claim 1.